The present invention relates to the field of novel genetically reassorted viruses, their use in veterinary medicine, a process for their preparation, vaccines prepared from them and methods of treatment of animals using them.
Equine influenza is a disease of horses of considerable veterinary and commercial significance. It is caused by RNA-containing equine influenza viruses, which belong to the family Orthomyxoviridae. Equine influenza virus, and indeed human influenza virus, each contain a segmented genome (Rohde et al, Virology (1977) 79,393-404 and Palese & Schulman, J.Virol. (1976) 17, 876-84). Genomic segmentation enables the discrete RNA segments of two parent viruses to reassort or recombine resulting in the production of progeny bearing RNA segments from both parents. Genetically reasserted viruses may be obtained by infection of for example hen eggs with two genetically distinct strains or sub-types of a particular virus, whereby the respective nucleic acid of the two parent viruses recombines to form a genome different from that of either parent but possessing characteristics from each of them.
In consequence of the unavailability of a reliable and effective chemotherapeutic agent to treat equine influenza, it has been customary in recent years to vaccinate horses against the disease. However, there is no cross-immunity between the two serotypes of equine influenza (Eq 1 and Eq 2). As a result of this, and because new strains of each serotype continually emerge and there is variable cross-immunity between all strains of the same serotype, it is desirable to be able readily to produce a vaccine for each of the various strains.
Traditionally, viruses that are used for influenza vaccine production are grown in fertile hen eggs. This technique is, however, costly and labour intensive especially because of the difficulty of purifying the resultant virus particles after they have been in contact with egg protein. Insufficient purification can cause immunogenic reactions in the vaccinated animal.
It has been disclosed (U.S. Pat. No 4,009,258 (E. D. Kilbourne, Mount Sinai School of Medicine); E. D. Kilbourne, Proc. Nat. Acad. Sci. 75(12) 6258; M. Baez et al, J. Infect Diseases 141(3) 362) that reasserted viruses may be obtained from the human influenza strain A/Puerto Rico/8/34 and other influenza viruses, the said reassortants giving especially high yields when grown in eggs.